The present invention relates to a method and a circuit for generating a reference voltage without thermal drift and of relatively low value, i.e. markedly lower than the voltage of a base-emitter junction (Vbe).
In many systems and particularly in monolithically integrated systems, it is necessary to implement voltage references, that is circuits capable of generating a stable reference voltage, free of thermal drift. Commonly this is achieved by employing a so-called band-gap circuit. A band-gap circuit produces a voltage corresponding to the sum of one or several base-emitter voltages (Vbe), as of common bipolar junction transistors, and of a voltage proportional to the difference between two different base-emitter voltages, suitably amplified by a certain amplification factor K, so as to make the amplified difference voltage comparable with the voltage of one or several base-emitter junctions, in order to produce a desired reference voltage given by: EQU Vref=Vbe+K.delta.Vbe, where K&gt;1.
The .DELTA.Vbe term that is employed for compensating the thermal drift of a certain sign of the particular Vbe or sum of Vbe used, may suitably assume a thermal coefficient of opposite sign of the thermal coefficient of the Vbe term used. Therefore, the resulting reference voltage Vref that is produced may be stable in terms of temperature variations.
Commonly band-gap circuits produce a temperature compensated voltage Vref greater or equal to about 1.2V. On the other hand, in systems designed for operating with relatively low supply voltages, for example in battery powered portable instruments and apparatuses, the supply voltage may be relatively low, for example in the vicinity of 1.0V. This makes a correct operation of a normal band-gap circuit impossible.
Recently, a band-gap reference voltage generating circuit has been proposed which is capable of providing a regulated voltage of relatively low level, in the vicinity of 200 mV, which may be adjusted upward to higher levels. This makes the voltage reference circuit suitable also in battery powered systems with a supply voltage of just 1V. The circuit is described in the article entitled: "A Curvature-Corrected Low-Voltage Bandgap Reference", by Gunawan, Meijer, Fonderie, and Huijsing, 28 IEEE JOURNAL OF SOLID STATE CIRCUITS 667-670 (1993), the content of which is herein incorporated by express reference.
Such a known circuit adopts a compensating system of the nonlinearity of the temperature characteristics of a base-emitter junction (Vbe). Basically, the circuit employs a first circuit block for generating a current proportional to the absolute temperature (PTAT) and a second circuit block capable of generating a current proportional to a Vbe, plus a correction current for compensating the nonlinearity of the temperature coefficient of the Vbe. Thereafter, the sum of the two currents is converted to a voltage signal which is amplified by an output buffer. The circuit is relatively complex and generates a stabilized reference voltage of about 200 mV, with a supply voltage that may be as low as about 1V.
There remains a need or utility for a circuit capable of generating a reference voltage of a relatively low value (on the order of a few tens of mV) without thermal drift, which is relatively simple to realize.
This objective is fully met by the method and the circuit object of the present invention.
Basically, the method of the invention rests on the generation of a stabilized voltage in the form of a sum of a voltage equivalent to the difference between two different base-emitter voltages, which is advantageously represented by a suitably controlled intrinsic offset voltage of a pair of transistors that constitute an input differential stage of a buffer-configured, operational amplifier, and a preestablished fraction of a base-emitter junction voltage. A subdivision of a Vbe voltage is implemented by mirroring, in a certain ratio, a current proportional to a Vbe voltage and by converting the divided-down mirrored current into a divided-down Vbe voltage on a resistance. The voltage difference between two different base-emitter junction voltages to be summed with the divided-down portion of a Vbe voltage, in order to compensate in terms of temperature the resulting voltage sum, is obtained in the form of an intrinsic offset voltage, controlled through a local feedback loop, of a differential pair of transistors that form an input stage of an operational amplifier that practically works as an output buffer of the stabilized voltage produced by the circuit.
The stabilized voltage sum that can be generated by the circuit may be of several tens of milliVolts and may be freely scaled-down by the use of a resistive voltage divider.
The circuit may be powered with a voltage of about 1V without jeopardizing its operation. Therefore the circuit is particularly useful in low voltage, battery powered systems.